Intermittent rivers affect global CO2 emissions

Press release published by Irstea / UGA

The results of the first global study of the contribution that intermittent rivers make to carbon cycling

A collaborative study involving 94 international partners was led by researchers from IRSTEA, the Laboratoire d’écologie alpine / OSUG (LECA) [1], the University of the Basque Country and the Leibniz-Institute of Freshwater Ecology and Inland Fisheries. This research, published on May 21st in Nature Geoscience, represents the first global study of the contribution that intermittent rivers make to carbon cycling.

Quantification and analyses of the plant litter deposited along the dry beds of 212 rivers distributed across the globe showed high levels of O2 consumption and CO2 emissions upon short-term simulated rewetting events. Associated drivers including climatic variables, flow regimes and geomorphological variables were also identified. These results highlight the need to incorporate intermittent river ecosystems into further studies exploring the contribution of inland waters to carbon cycling at the global scale.

Intermittent rivers, as their name suggests, sometimes stop flowing and can dry completely. Although far less studied than permanent rivers, they could represent half of the world’s river network and, in response to climate change and increasing water demands, may come to dominate the landscape in some regions.

When a river ceases flowing, terrestrial plant litter, mostly leaves and wood from the adjacent riparian zone, falls and accumulates in dry river beds. The type and amount of litter varies, depending on climate, riparian vegetation, the width of the river channel, the duration of the dry period and the river’s flow regime. However, scant attention has previously been paid to this plant litter’s fate when a river resumes flowing.

To shed light on what happens to the litter during rewetting and to explore the contribution that intermittent rivers make to the global C cycle, the 1000 Intermittent Rivers project2 -a unique collaborative network- investigated the quantity and quality of terrestrial plant litter that accumulated during dry periods in 212 intermittent rivers across 22 countries. Sub-samples of leaf litter collected by this international research consortium were analysed at Irstea-Lyon and the Laboratoire d’Ecologie Alpine following standardized assays to simulate short-term (24-hours) rewetting events. High respiration rates were measured, reflecting the reactivation of microbial communities within the litter. In turn, this activity released substantial quantities of CO2 into the atmosphere. A rough extrapolation indicates that estimates of daily CO2 emissions from inland watercourses could rise by between 7 and 152% if data from intermittent rivers are added to existing data from perennial rivers, and one rewetting event could contribute up to 10% of this increase.

The results suggest that the exclusion of intermittent rivers leads to notable underestimation of the contribution of the world’s river networks to the release of CO2 into the atmosphere. The next step is to incorporate intermittent rivers into global models of litter decomposition and global carbon cycling in inland waters.